1. Field of the Invention
The present invention relates to a chip resistor and a method for making the same, and particularly to a chip resistor having barrier layers and a method for making the same.
2. Description of the Related Art
FIG. 1 shows a schematic cross-sectional view of a conventional chip resistor. The chip resistor 1 is a passive device soldered on an integrated circuit board, and is used for providing resistance. The chip resistor 1 includes a substrate 11, a pair of main upper electrodes 12, a pair of bottom electrodes 13, a resistive film 14, a first protective coat 15, a second protective coat 16, a pair of side electrodes 17, a pair of first plated layers 18, and a pair of second plated layers 19.
The substrate 11 is made of an insulating material, an approximately rectangular plate, and has a back face 111, a pair of side faces 112, and a main face 113. The side faces 112 respectively extend upwards from two opposite sides of the back face 111. The main face 113 corresponds to the back face 111. The main upper electrodes 12 are conductively disposed on the main face 113 of the substrate 11, and are separate from each other. Each main upper electrode 12 has an inner side face 121, an outer side face 122, and an inner end portion 123. The outer side face 122 of the main upper electrode 12 is aligned with the side face 112 of the substrate 11.
The bottom electrodes 13 are conductively disposed on the back face 111 of the substrate 11, and are separate from each other. Each bottom electrode 13 has an outer side face 132. The outer side face 132 of the bottom electrode 13 is aligned with the side face 112 of the substrate 11, such that the main upper electrodes 12 and the bottom electrodes 13 are symmetrical to each other.
The resistive film 14 has a predetermined resistance and is disposed on the main face 113 of the substrate 11, and is disposed in a region between the inner side faces 121 of the main upper electrodes 12. The resistive film 14 extends over the main upper electrodes 12, such that two end portions of the resistive film 14 overlap with the inner end portions 123 of the main upper electrodes 12. The first protective coat 15 is made of a cuttable insulating material, and covers the resistive film 14, such that the resistive film 14 is isolated from the outside environment. The second protective coat 16 is made of an insulating material, and covers the first protective coat 15 and part of the main upper electrodes 12, such that the resistive film 14 and the first protective coat 15 are isolated from the outside environment.
The side electrodes 17 are made of a conductive material. Each side electrode 17 is formed on the side face 112 of the substrate 11, the outer side face 122 of the main upper electrode 12, and the outer side face 132 of the bottom electrode 13, for electrically connecting the main upper electrode 12 and the bottom electrode 13. The first plated layers 18 are nickel layers, and each first plated layer 18 covers the main upper electrode 12, the bottom electrode 13, and the side electrode 17. The second plated layers 19 are tin layers, and each second plated layer 19 covers the first plated layer 18. The second plated layers 19 and the first plated layers 18 are formed by electroplating.
The disadvantage of the conventional chip resistor 1 is described as follows. In an environment with high sour gas and high corrosive gas, the corrosion gas easily penetrates the chip resistor 1 through the interfaces between the second protective coat 16 and the first plated layer 18 and between the second protective coat 16 and the second plated layer 19, and chemically reacts with silver or copper in the main upper electrode 12 to generate silver sulfide or copper sulfide, thus changing the resistance value. More seriously, an open-circuit may be formed, which will paralyze the system where the chip resistor 1 is located.
Therefore, it is necessary to create a chip resistor that solves the above problem and a method of making the same.